1. Field of the Invention
The present invention relates to an optical burst switching (OBS) network and, more specifically, to a method for matching a transmission rate of burst data input into an optical burst switching network and a transmission rate of burst data output.
2. Description of the Related Art
Generally, when an optical signal is transmitted and received through an optical link, an electrical switch is used. However, the electrical switch must first convert the optical signal into an electrical signal, in order to process the optical signal transferred. Then, it must convert the processed electrical signal back to an optical signal prior to transmission. So, the electrical switch additionally needs an optoelectric converter to convert the optical signal into the electrical signal and electrooptic converter to convert the electrical signal into the optical signal, resulting in an increased cost.
In order to solve such a problem, there was proposed an optical burst switch, which does not convert the transferred optical signal into the electrical signal but processes the optical signal directly. Hereinafter, an optical burst switching network using an optical burst switch will be described.
In the optical burst switching network, generally, internet protocol (IP) packets input in an optical domain are gathered as burst data in an edge node, and such burst data is routed by way of a core node depending on destinations of the packets or Quality of Services (QoS) and then sent to the destination nodes. Further, a burst control packet (BCP) and the burst (optical) data are respectively transmitted on different channels and at an offset time. That is, the burst control packet is transmitted earlier than the burst data by the offset time and the burst control packet previously reserves a path through which the burst data is transferred, so that the burst data can be transmitted through the optical network at a high speed without being buffered. Hereinafter, a procedure for transmitting the optical data will be described with reference to FIG. 1.
FIG. 1 is a view showing nodes for transmitting and receiving, or switching, burst data in an optical burst switching network. Hereinafter, a procedure to transmit the burst data in the optical burst switching network will be described.
When a node A 100 which is an edge node receives IP packets, it gathers the IP packets and forms the burst data. Edge nodes 100, 106 and 108 gather the IP packets, and form and transmit the optical burst packet. Further, the edge nodes 100, 106 and 108 receive the optical burst data and divide it into the IP packets. The core nodes 102 and 104 optically switch the burst data. When a desired size of burst data is generated, the node A 100 generates a burst control packet (BCP) and transmits it to the node B 102, a core node. The node A 100 transmits the burst data to the node B 102 after an offset time. The BCP includes information on a destination address of the burst data, a source address of the burst data, a size of the burst data, QoS, and an offset time, and other such information known in the art.
The node B 102 uses the BCP to identify the destination address of the burst data which will be received, determine an optical path and schedule time for the optical switching. In the node B 102, while the burst control packet is converted from the optical signal to the electrical signal or vice versa, the burst data goes through the optical path by the optical switching without optoelectric conversion. The node B 102 can switch the burst data to the node D 106 or the node C 104 depending on whether the destination of the burst data transmitted from the node A 101 is the node D 106 or the node E 108.
The procedure where the node B 102 transfers the burst data transmitted from the node A 100 to the node D 106 or the node E 108 was explained in the above description. However, the node B 102 may either be a destination of the burst data generated in the node A 100, or may generate burst data to be transmitted to the node D 106 or the node E 108. In other words, the node B 102 may be a core node or may have a function of an edge node.
In case of the node B, however, there occurs a case where both the node A and the node C attempt to send burst data simultaneously to the node D. In this case, since the node B cannot transfer both transferred burst data from nodes A and C to the node D at the same time, the node B selects the burst data from one of the nodes A and C and transmits that burst data first. Thereafter, the burst data from the node that was not selected is transmitted after a certain time, so that the burst data can be prevented from being lost.
Data input to the optical burst switching network can be generally divided into two types: data of constant bit rate (CBR), such as voice data, and data of variable bit rate (VBR), such as packet data. The data of constant bit rate should be received in the receiving-end with the same bit rate as that of data transmitted in the transmitting-end.
FIG. 2 is a view showing a bit rate of data received in an optical burst switching network. Referring to FIG. 2, an optical burst switching network includes an ingress edge node (node A), an egress edge node (node D) and core nodes (node B, node C) that are also edge nodes. The nodes constructing the optical burst switching network have their own natural frequencies (the number of clocks per unit time). As shown in FIG. 2, natural frequencies of node A, node B, node C and node D are fa, fb, fc and fd, respectively. Since phase may be represented by integrating the frequency, phase, frequency and the number of clocks per unit time are all regarded as having the same meaning, hereinafter.
Referring to FIG. 2, it is noted that phases of data output from the node A, data output from the node C, data input into a damper (not shown) constructing the node D, and data output from the node D are different from one another. As described above, data of a constant bit rate should be transmitted at a fixed rate. However, a bit rate of data input into the optical burst switching network is different from that of data output therefrom. Such a problem is due to the characteristic of the optical burst switching network as well as differences of the natural frequencies of nodes. That is, the optical burst switching network does not transmit circuit data but transmits data by burst consisted of a plurality of packets. Accordingly, the packet data that are received first should be on standby (delayed) until other data is received. Further, in case that the core node has no link available for transmission of the burst data, the burst data is placed on the standby in the core node for a certain time. As such, the data transmitted to the optical burst switching network is delayed for various reasons, and the constant bit rate cannot be guaranteed for such reasons.